This invention relates to a method of releasably connecting a blade to a single wheel plate of a centrifugal blasting wheel without using a separate stop-member to prevent outward radial movement of the blade. The invention also relates to a blade adapted to be releasably connected to the single wheel plate and a centrifugal blasting wheel having a blade releasably connected in accordance with the teaching of the method aspect of the invention.
Centrifugal blasting wheels have been known for some time. In essence, abrasive material is supplied to a rotating blasting wheel, usually from a central area of the wheel. The abrasive is directed against an object to be cleaned by a plurality of blades which radiate outwardly from the central area of the blasting wheel and which rotate as the wheel rotates. Typically, the blades are connected to one or two annular wheel plates. Many methods have been devised to connect the blades to one or more wheel plates.
One such connecting method is disclosed in U.S. Pat. No. 2,869,289. In accordance with that prior art method, a connecting member extends from one side edge of the blade and the connecting member is slidable into a corresponding channel in the face of the annular wheel plate. In one form, the connecting member in the prior art was a dovetail-shaped member in cross-section and the cross-section remained a uniform size throughout. In essence, the side faces of the dovetail connecting member were in radial planes. Also, the side faces of the dovetail connecting member corresponded to side faces in the corresponding channel in the annular wheel plate.
One of the difficulties with this prior art connecting method was that in order to prevent the blade from moving radially outwardly some form of stop member was used which was inserted into or through the wheel plate to prevent the blade from moving radially outwardly. As the wheel rotated, the blade moved radially outwardly until it abutted against the stop member and the stop member prevented further outward radial movement. The difficulty with the stop members was that they tended to wear more quickly than other parts of the wheel and blades and, therefore, had to be replaced more frequently.
In addition to stopping the blade from further outward radial movement, the stop members is used to properly position the blades so that all of the blades in the wheel, which are substantially identical, are positioned the same distance from the centre of wheel so that the mass of the wheel is balanced and the wheel rotates properly because the stop member wears more easily than other parts of the wheel, if these pins are not changed on a timely basis, one blade may advance further into the channel than intended. If the blade diametrically opposite to this blade which has advanced more than planned into the channel has not so advanced, there will be a mass imbalance on the wheel which will cause problems with proper rotation.
Because the blades were typically cast and not ground, the tolerances were such that there was a gap between the two side faces. In order that the blade could be slid readily into the channel, it was necessary that there be some gap between the two side faces. Therefore, another difficulty with the prior art connecting methods was that, during operation, abrasive shot would become lodged in the space or gap between the side face of the connecting member of the blade and the side face of the channel in the wheel plate.
After the blade had been inserted and the apparatus had been in operation for some time, the abrasive which became lodged between the blade and the channel made the blade extremely difficult to remove. It is necessary to remove the blades relatively frequently because the blades wear more frequently than other parts of the wheel because of a constant contact with abrasives.
In order to remove a prior art blade when abrasive is lodged between the blade and the channel, it was necessary to apply a sufficient striking force against the outer end of the blade, such as with a hammer, to move the blade inwardly along the entire length of the channel. This task is difficult and dangerous. It is dangerous because the blades are relatively hard and brittle. As a result of the hammering, often a chip will be chipped from the end of the blade. If the chip is not recovered there is a possibility that it will find its way into the other working parts of the wheel. Because it is so hard, the chip may damage other parts of the blasting wheel.
Another difficulty with the prior art connecting blades is that, because of the tolerances as a result of casting, the channel slot is larger than the blade connecting member. This results in a sloppy fit between the blade and channel. This sloppiness means that the blades do not always square with the face of the wheel plate and can be two or three degrees off square. This means that the shot is not thrown properly in a plane parallel to the face plate and all of the abrasive will not correctly strike the object to be cleaned.